Non-oriented electrical steel sheet is known to become minimum in core loss at a grain size of 150 μm or so. In the finish annealing process, the crystal grains are therefore grown. For this reason, from the viewpoint of product core loss or from the viewpoint of the simplification of production and raising productivity, steel sheet with better crystal grain growth characteristics in the finish annealing is therefore desired.
On the other hand, electrical steel sheet is stamped by the consumer for use for producing cores. The smaller the grain size, the better the stamping precision in the stamping operation. The grain size is therefore, for example, 40 μm or less.
Therefore, sometimes the measure is taken of shipping the product sheet with the small grain size, then having the consumer stamp it, then for example perform stress relief annealing at 750° C.×2 hours or so to grow the crystal grains.
In this case, consumers increasingly demand product sheets with good growth potential of the crystal grains even with low temperature, short time stress relief annealing so as improve productivity.
One of the primary factors that obstruct crystal grain growth is the inclusions finely dispersed in the steel. It is known that the greater the number of inclusions contained in the product and the smaller their size, the more the crystal grain growth is obstructed.
That is, as indicated by Zener, the smaller the r/f value as expressed by the spherical equivalent radius r of the inclusions and the volume fraction f of the inclusions in the steel, the worse the crystal grain growth. Consequently, in order to improve the crystal grain growth, the number of inclusions must, of course, be reduced and it is critical to increase the size of the inclusions.
As fine inclusions that obstruct crystal grain growth of the non-oriented electrical steel sheet, oxides such as silica and alumina, sulfides such as manganese sulfide, and nitrides such as aluminum nitride and titanium nitride are known.
In order to eliminate these fine inclusions or decrease them to the necessary and sufficient level, it is self-evident that the purity should be increased at the molten steel stage.
However, eliminating these fine inclusions or decreasing them to the necessary and sufficient level by increasing the purity at the molten steel stage is not preferable since an increase in the steelmaking cost is unavoidable.
Therefore, as other methods, several methods are known of adding various elements to the steel ensure to render the inclusions harmless.
For the oxides, technological advances have made it possible to eliminate and render harmless oxides at the molten steel stage by adding a sufficient amount of the strong deoxidizing element Al and allowing sufficient time for the flotation and removal of oxides.
For the sulfides, for example, as disclosed in Japanese Patent Publication (A) No. 51-62115, Japanese Patent Publication (A) No. 56-102550, Japanese Patent Publication (A) No. 59-74212, Japanese Patent No. 3037878, etc., the method is known of adding rare earth elements (below called “REM”), which are desulfurization elements, etc. so as to increase the size of S inclusions and render them harmless.
Further, for nitrides, as disclosed in Japanese Patent No. 1167896 and Japanese Patent No. 1245901, the method for rendering N harmless as coarse inclusions by the addition of B is well known.
However, even if using the above stated methods to eliminate oxides, sulfides, and nitrides of non-oriented electrical steel sheet or increase the size of the inclusions to render them harmless and then perform the finish annealing or stress relief annealing, the crystal grains will partially vary in growth and fine crystal grains and coarse crystal grains will be mixed together—sometimes leading to poor core loss.
The cause, it has been found, is the fine titanium carbides (below called “TiC”), derived from the Ti and C which had been in solid solution at the stage of the finish annealing or stress relief annealing, precipitating at parts of the product sheet and obstructing the growth of the crystal grains. This will be explained specifically below.
Non-oriented electrical steel sheet is often treated by finish annealing or stress relief-annealing at a comparatively low temperature of 1000° C. or less. In particular, stress relief annealing is performed at 750° C. or so or at a further lower temperature to prevent wear of the surface coating of the product sheet.
Therefore, in order to sufficiently grow crystal grains at such a low temperature, it is necessary to perform the annealing over a long time of 1 hour or more.
With annealing at this low temperature and long time, it is difficult to control the temperature of the product sheet to become uniform over the entire surface at all times. Parts of the product sheet become lower in temperature, while other parts become higher in temperature, i.e., a variation often occurs in the temperature distribution.
Incidentally, when TiC precipitates in electrical steel, it has been learned from separate studies that it precipitates within a range of 700 to 800° C., particularly actively precipitates at 750° C. or less.
Consequently, in annealing at a low temperature over a long time, at the parts where the temperature of the product sheet becomes relatively high, the precipitation temperature of TiC is exceeded, so TiC does not precipitate. Further, since these parts are high in temperature, the crystal grain growth rate is also fast. Therefore, the crystal grains of these parts become coarse in size.
On the other hand, at the parts where the temperature of the product sheet becomes relatively low, the temperature is less than the precipitation temperature of TiC, so TiC precipitates during the annealing.
In particular, the TiC produced under a low temperature, due to the low temperature, cannot grow to TiC of a sufficient size and becomes fine, so obstructs crystal grain growth during annealing over a long time.
Since the TiC particles precipitating in this case are fine, even if the amount of Ti and the amount of C contained in the steel are high ones of several ppm, sometimes a number of TiC particles sufficient for obstructing crystal grain growth will precipitate.
Furthermore, at the parts where the temperature of the product sheet is relatively low, due to the low temperature, the growth rate of the crystal grains itself is slow and therefore the effect of the fine TiC particles obstructing crystal grain growth becomes stronger. Therefore, the crystal grains do not sufficiently grow and remain fine.
In this manner, the reduction of the annealing temperature or the unavoidable variation in the annealing temperature causes variation in presence of TiC in the electrical steel sheet and consequently variation in crystal grain growth in the electrical steel sheet.